Pixhawk AC 3.6.9 altitude telemtry data

i use RadioLink AT9S with PRM-02 telemetry on my Pixhawk
when i switch to loiter mode my radios telemetry show GPS altitude and its not helping me to understand relative altitude .
how i can change it to show or Pixhawk send relative altitude to PRM-02 telemetry ?

I had this issue, think its covered in this thread near the end:

Thank you
It was really helpful

Solution is here